A centrifugation device, often referred to as centrifuge, is used to separate laboratory specimen samples into component parts. For example, blood includes liquid, cells and other components. By spinning the specimen samples within the centrifuge at high velocity, such as 3,000 revolutions per minute (RPM), the samples are separated into the respective component parts based upon relative specific gravity of the components within the samples. When a sample includes components with differing specific gravities, the heavier components are forced to the bottom of the centrifuge tube when processed within the centrifuge. The separation of samples into component parts allows scientists and laboratory technicians to determine the relative quantities, through both qualitative and quantitative analysis, of the component parts within the samples, as well as providing an opportunity to further process the respective component parts.
A centrifugation device includes a rotor and a drive system that causes the rotor to rotate. The rotor is typically disk-shaped and intended to support specimen samples at its outer peripheral edge. The rotor possesses sufficient centripetal properties to retain the specimen samples against elevated gravitational forces generated when the rotor is rotated at high velocity.
With conventional centrifuge designs, the rotor support mechanism, excluding the drive motor (referred to herein as the gyro), can be little more than a bearing to support the rotor and a shaft connecting the rotor to the prime mover. If this rotor is sufficiently balanced, no apparent vibrational energy is translated to the support chassis as the rotor approaches operational speeds, referred to as “slew” speeds. However, in actual practice, there is at least some variation in specimen volume, and this mass differential alone can cause rotor imbalance to a greater or lesser extent. This rotor imbalance can cause significant vibration within a conventional centrifugation device.
To further explain the effect of an unbalanced centrifuge, FIG. 1 illustrates an exemplary trapezoidal velocity-time plot for a conventional centrifuge with time on the X-axis and velocity on the Y-axis. As can be seen from FIG. 1, the centrifuge rotor velocity appears as an ascending slope A with the angle dependent on acceleration rate after the centrifuge is activated. The plot then transitions to a flat line B as the rotor maintains constant velocity for a period of centrifugation time. The flat line B is referred to as the slew speed for the centrifugation process. The plot then has a descending slope C with the angle dependent on deceleration rate after the centrifuge is deactivated.
FIG. 1 also illustrates a phenomenon known as resonance. Resonance occurs within all rotational centrifugation devices as a result of forces which act upon the rotor as it speeds up and slows down. All rotors transition through resonance as they accelerate and likewise transition through resonance again as they decelerate.
As can be seen from FIG. 1, resonance occurs on the ascending slope A during acceleration and on the descending slope C during deceleration. Resonance is a function of the total rotational mass, any imbalanced mass, and the spring rate of the mechanical coupling (e.g., gyro) between the rotor and the chassis. Conventional rigid rotor/chassis couplings produce relatively high resonance speeds, typically occurring close to slew speeds.
In centrifuge rotors, energy is a function of the square of the velocity. As such, twice the velocity equals four times the energy. Accordingly, the high resonance speeds of conventional centrifugation devices produce significant accumulated energy at resonance and impart significant vibration to the chassis of the centrifuge.
Because of the high speeds and high accumulated energy at resonance, conventional centrifugation devices are particularly sensitive to unbalanced loading. An unbalanced load within a conventional centrifuge can result in significant imbalance of the forces at resonance. The additional forces caused by the unbalanced load at resonance can result in the rotor/gyro systems becoming violently unstable during resonance and cause them to physically crash into internal mechanical stops or exceed preset vibration limit switches, thus removing drive power.
FIGS. 2A-2C illustrate force vectors associated with centrifugation devices during an acceleration sequence. As can be seen from FIG. 2A, when counter-clockwise acceleration begins, a force vector D is in relative alignment with a motion vector E. The force vector D represents radial forces (e.g., stress) on the rotor and the motion vector E represents rotational movement (e.g., strain) of a centrifuge rotor. Throughout acceleration, the direction of the vectors changes continually and the motion vector E begins to lag behind the force vector D. This lag results from the fact that the force vector D is continually applied in a new direction throughout acceleration, which is ahead of the induced motion vector E. The angle quantifying the lag of the motion vector E behind the force vector D is termed phase angle. The phase angle is zero at zero RPM, approximately 90 degrees at resonance, and approximately 180 degrees at high speeds. The frequency of rotation divided by the resonant frequency is generally termed the frequency ratio. FIG. 2B shows that the motion vector E lags the force vector D by approximately 90 degrees at resonance. This 90 degree lag results in the violent instability of conventional centrifuge devices if loaded with an unbalanced load. FIG. 2C shows that the motion vector E lags the force vector D by approximately 180 degrees at slew speeds. When the vectors are at equal magnitude and opposite in direction, they effectively cancel one another and the centrifuge rotates smoothly without imparting vibration to the chassis. Any change in magnitude or phase angle may result in some vibration imparted to the chassis. Accordingly, as described above, if a conventional centrifuge is imbalanced, it may never reach slew speeds due to the high rotational energy accumulated by the time the rotor reaches resonance.
A previous solution has employed a flexible coupling between the prime mover and the rotor to allow lateral movement of the center of mass to automatically balance an unbalanced centrifuge. Frictional damping is used to limit lateral movement of the rotor. However, this causes vibration in the rotor and chassis of the centrifuge device, which is undesirable.
Accordingly, there exists a need to provide a device and system capable of automatically balancing centrifuge rotors that does not require load balancing of tubes placed in the centrifuge, and particularly for automatic critically-damped inertial mass centering of a centrifuge rotor to accommodate unbalanced loads without imparting significant vibration on the centrifuge chassis and without lateral frictional damping.